Ion implantation is a process used to dope impurity ions into a semiconductor substrate. In accordance with the process, an ion beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and mass of the ions generated in the source chamber. A precise doping profile in the substrate is critical to proper device operation. One or more types of ion species may be implanted in different doses and at different energy levels to obtain desired device characteristics.
During implantation or other workpiece processing, accelerated ions will sputter materials off any impinged surface eroding the surface.
Ion sources that generate the ion beams used in existing implanters are typically called arc ion sources and can include heated filament cathodes for creating ions that are shaped into an appropriate ion beam for wafer treatment.
U.S. Pat. No. 5,497,006 describes an ion source having a cathode and anti-cathode (repeller) disposed in the gas confinement chamber. The ion source further includes a solid aluminum block for retaining the gas confinement chamber with a support portion of the base. The cathode is a tubular conductive body and endcap that partially extends into the gas confinement chamber. A filament is supported within the tubular body and emits electrons that heat the endcap through electron bombardment, thermionically emitting the ionizing electrons into gas confinement chamber.
U.S. Pat. No. 8,253,334 also discloses a cathode sub-assembly comprised of a retainer, a cathode and collar, each of which has smooth unthreaded surfaces that slidably engage each other. A shield serves to hold the sub-assembly in a support plate. The cathode projects from the sub-assembly into an arc chamber with a tortuous path created therebetween for passage of the plasma flow.
U.S. Pat. No. 5,763,890 also discloses an arc ion source for use in an ion implanter. The ion source includes a gas confinement chamber having conductive chamber walls that bound a gas ionization zone. The gas confinement chamber includes an exit opening to allow ions to exit the chamber. A base positions the gas confinement chamber relative to structure for forming an ion beam from ions exiting the gas confinement chamber.
U.S. Patent Application 2011/0156570 also discloses a cathode assembly for use in an ion implanter. The ion source includes a gas confinement chamber having a filament clamp assembly. The filament clamp assembly has a pair of bifurcate clamps to hold the connecting leads of a filament within a cavity of a cathode of a separate cathode assembly. The filament clamp assembly is mounted on the insulator block in self-aligning relation. The cathode assembly has a tungsten cathode with an internal cavity to receive the filament that is secured within a retainer shield made of tungsten, molybdenum and graphite by a threaded graphite cylindrical collar.
U.S. Pat. No. 7,750,313 proposes to improve the plasma generation efficiency and gas use efficiency as well as to ensure a longer service life an ion source. As described, a cathode holder of tubular shape is inserted into an opening and positioned such that a surface thereof opposes or surrounds a side surface of a cathode. The cathode is held in the cathode holder so that a front surface of the cathode will be positioned on the same plane as, outward from, or inward from the inner wall surface. The cathode holder is also provided with a tubular first heat shield surrounding the cathode with a space provided between the first heat shield and the cathode. A surface of the first heat shield is positioned to oppose or surround the side surface of the cathode. A filament is provided at a rear end of the cathode and a gap between the cathode holder and the plasma generating chamber is filled with an electrical insulating material.
U.S. Pat. No. 9,425,023 discloses a cathode that extends outward from the inside of an arc chamber in an axial direction and that emits a thermal electron into the arc chamber, a thermal reflector with a cylindrical shape provided around the cathode in a radial direction and extending in the axial direction, and a narrow structure configured to narrow a width in the radial direction of a gap between the cathode and the thermal reflector at a given position in the axial direction.
U.S. Pat. No. 9,659,755 discloses a plasma generator including an arc chamber having a plasma generation region in which plasma is generated in the inside thereof; a magnetic field generator is configured to apply a magnetic field to the plasma generation region; and a cathode is configured to extend in an axial direction along an applying direction of the magnetic field to the plasma generation region and is provided with a cathode cap that emits thermal electrons at a front end thereof. The cathode cap protrudes toward the inside of the arc chamber in the axial direction and has a shape of which a width in the radial direction perpendicular to the axial direction becomes smaller toward the inside of the arc chamber.
Other sources may drive RF, microwave or electron beam discharges to generate desired ions. These sources generate plasma densities 10-100 times lower than arc ion sources and are typically used with source materials that have low ionization potentials (species that are easy to ionize) or when the source chamber includes large ion extraction areas. Cold ion sources, such as shown in U.S. Pat. No. 6,975,072, can have source materials made of relatively low temperature materials, such as stainless steel, copper or aluminum. Hot sources, such as, arc ion sources expose the source chamber walls to an arc plasma at temperatures of several tens of thousands of degree Celsius and high thermal power densities which have required source components of prior art implanters to be made of high temperature, so called refractory, materials, such as molybdenum, tantalum or tungsten.
Accordingly, it is an object of the invention to improve the life performance of a cathode assembly during ion beam operation.
It is another object of the invention to provide an indirectly heated cathode (IHC) ion source assembly that can be used in all ion source arrangements.
It is another object of the invention to provide an indirectly heated cathode (IHC) ion source assembly that can be fitted in OEM implant systems as well as iTS ion sources.
It is another object of the invention to provide an improved cathode assembly structure for ion implanters that reduces the number of parts, improves the thermal electron emission, reduces the cathode erosion and reduces ion implanter downtime.